It is not illegal to use hydrocarbon refrigerants in all cases.
It is illegal to use them in 18 states (Arkansas, Arizona, Connecticut, Florida, Idaho, Indiana, Iowa, Kansas, Louisiana, Maryland, North Dakota, Oklahoma, Texas, Utah, Virginia, Wisconsin, Washington, and the District of Columbia) for any automotive application.
It is illegal to use them in r12 systems.
It is okay to use them on HFC134 systems or systems properly (legally) converted to HFC134 outside of the 18 states.
This means if you live outside the 18 states you must first convert your old r12 system to HFC134. Only then can you convert to a hydrocarbon system.
95 E320 Cabriolet, 131K